Note: Descriptions are shown in the official language in which they were submitted.
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WO 2011/144461 Al
DESCRIPTION
POROUS CARBON WITH HIGH VOLUMETRIC CAPACITY, FOR
DOUBLE-LAYER CAPACITORS
The present invention relates to an activated, porous
carbon having a defined specific BET surface area and a
defined pore distribution that may be used as an
adsorption material or as an electrode, and
particularly as an electrode in a double-layer
capacitor.
Because of its high porosity, activated carbon, or
activated charcoal, is frequently used as a an
adsorption material, particularly to remove unwanted
colouring agents, flavouring substances and/or odorants
from gases and liquids, for example in waste water
treatment or air purification. In such cases, the
activated carbon may be in granulate, powder or pellet
form depending on the particular application.
Besides this use, and also because of its high
porosity, activated carbon also lends itself well to
use as an electrode material, for example in double-
layer capacitors, which are also called supercapacitors
and are becoming increasingly important due to their
high energy density. Such double-layer capacitors are
made with two electrodes, separated from one another by
a separator, and each of which being coated with
electrolyte. In order to be able to store high energy
densities, double-layer capacitors need electrode
material with the highest possible volumetric capacity.
However, the volumetric capacity cannot be increased by
increasing the specific surface area of the electrode
or carbon material indefinitely, because increasing the
specific surface area simultaneously reduces the
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density of the activate carbon, thus again resulting in
a loss of volumetric capacitance.
The document DE 101 97 141 B4 describes the use of an
alkali-activated carbon as the electrode in an
electrical double-layer capacitor, wherein the alkali-
activated carbon contains pores of a first group of
pores having a pore diameter D not exceeding 2 nm,
pores of a second group of pores having a pore diameter
D greater than 2 nm but not exceeding 10 nm, and pores
of a third group of pores having a pore diameter D
greater than 10 but not exceeding 300 nm, wherein the
volume of the pores in the first group of pores
constitutes more than 60% of the total volume of all
pores of the first, second and third groups combined,
and the volume of the pores in the second group of
pores constitutes more than 8% of the total volume of
all pores of the first, second and third groups
combined, and wherein the volume of the pores in the
first group of pores constitutes is greater than 0.10
to 0.44 ml/g, and the volume of the pores in the second
group of pores is greater than 0.01 to 0.20 ml/g. The
specific surface area of the activated carbon is about
500 to 1,150 m2/g. Whereas the pores in the first group
of pores are intended particularly to promote the
development of electrical capacitance, the pores in the
second group of pores are intended to ensure that ions
are diffused in the carbon and that the carbon is
impregnated with electrolytic solution, and the pores
in the third group of pores are intended to promote the
impregnation of the carbon with electrolytic solution.
In this context, the capacitance density or volumetric
capacitance of a double layer capacitor produced using
electrodes made from such a carbon should become
greater as the fraction of pores in the first group of
pores is increased up to a value of 80% relative to the
total number of all pores in the carbon, but if the
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fraction of pores in the first group of pores is
increased above 80% relative to the total number of all
pores the volumetric capacitance should begin to fall
again. A double layer capacitor produced using
electrodes made from such a carbon should have a
capacitance density or volumetric capacitance from 30
to 41 F/cm3 carbon. However, the energy density that
can be stored by double layer capacitors produced using
electrodes made from such a carbon is in need of
improvement.
Accordingly, in order to produce double lairer
capacitors that are capable of storing greater energy
density, an activated carbon that is capable of lending
double layer capacitors increased volumetric
capacitance is desirable.
The present invention relates to
. an easily producible, activated porous carbon
having greater volumetric capacitance than the carbons
known from the prior art, and which is therefore very
well suited for use as electrode material in double
layer capacitors and may be used to produce double
layer capacitors that are capable of storing a
particularly high energy density.
More particularly, the invention relates to
an activated, porous carbon having a specific BET
surface area between 1,400 and 1,900 m2/g, wherein at
least 80% of all pores in the carbon have an average
diameter between 0.3 and 0.9 rim.
This invention is based on the surprising discovery that
an activated, porous carbon with a defined BET surface
area that is made up exclusively, or at least
practically exclusively, of micropores, but no or
almost no mesopores or macropores, that is to say
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activated, porous carbon having a specific BET surface
area between 1,400 and 1,900 m2/g, wherein at least 80%
of all pores in the carbon have an average diameter
between 0.3 and 0.9 nm, exhibits particularly high
specific and volumetric capacitances and, when used as
electrode material in a double layer capacitor for
example, results in layer capacitors that are capable
of storing a particularly high energy density.
The specific surface area of the activated carbon cited
in the preceding text is measured according to the
present patent application with a device for measuring
surface area and pores with the brand name AUTOSORB-6
that is marketed commercially by the company
QUANTACHROME GmbH & Co. KG, Odelzhausen, Germany. With
this instrument, nitrogen isotherms are measured at 77
K and the samples for measurement are baked out for 1
hour in a vacuum at 350 C. Analysis is carried out
using the software AS1 Win, Version 2.01, which is also
marketed by QUANTACHROME GmbH & Co. KG.
In order to measure the pore radius distribution, from
which the fraction of the total number of all pores
having an average diameter between 0.3 and 0.9 nm is
determined according to the present application, a
measuring device for surface area and pore analysis is
used that has the brand name NOVA 2200, and is also
marketed commercially by the company QUANTACHROME GmbH
& Co. KG, Odelzhausen, Germany. With this instrument,
carbon dioxide isotherms are measured at 0 C, and the
samples for measurement are baked out for 1 hour in a
vacuum at 350 C. The average pore radii are calculated
according to the "Nonlocal Density Functional Theory"
(NLDFT) and the Monte Carlo method.
According to the invention, at least 80% of all pores
of the carbon have an average diameter between 0.3 and
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0.9 nm. Especially high volumetric and also specific
capacitances are obtained particularly if at least 90%
of all pores, preferably at least 95% of all pores,
particularly preferably at least 99% of all pores and
most preferably all of the pores in the carbon have an
average diameter between 0.3 and 0.9 nm.
In a refinement of the inventive thought, it is
suggested that the activated, porous carbon may have a
total pore volume between 0.7 and 1.2 cm3/g, wherein in
particular activated, porous carbon having a total pore
volume between 0.7 and 1.0 cm3/g, and particularly
preferably having a total pore volume between 0.8 and
0.9 cm3/g exhibits particularly good properties for
technical application purposes. According to the
present patent application, the total pore volume is
measured with a measuring device for surface area and
pore analysis with the brand name AUTOSORB-6, which is
marketed commercially by QUANTACHROME GmbH & Co. KG,
Odelzhausen, Germany. With this instrument, nitrogen
isotherms are measured at 77 K and the samples for
measurement are baked out for 1 hour in a vacuum at 350
C. Analysis is carried out using the software AS1 Win,
Version 2.01, which is also marketed by QUANTACHROME
GmbH & Co. KG.
As was explained earlier, activated, porous carbon with
the stated specific surface area and pore
characteristics has particularly high specific
capacitance and particularly high volumetric
capacitance.
The specific capacitance of the carbon preferably lies
between 130 and 150 F/g, whereas the volumetric
capacitance of the carbon preferably lies between 80
and 100 F/cm3.
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The stated capacitances of the carbon, that is to say
the specific capacitance and the volumetric
capacitance, refer to the capacitance relating to a
single electrode produced from the carbon, which
according to the present invention is measured as
follows by galvanostatic cyclisation: electrodes in the
form of round pellets having a diameter of 10 mm and a
mass of 10 mg each are formed from the activated
carbon, after which the electrical capacitance thereof
is measured with a "Whatman" glass fibre separator
having a thickness of 30 pm at 2.3 V and a charge
current of 500 mA/g in a Swagelok cell with 1 M
tetraethyl ammoniumtetrafluoroborate in acetonitrile as
the electrolyte, and the specific capacitance and
volumetric capacitance are calculated therefrom.
The previously described, activated porous carbon may
particularly be produced by process based on alkali
activation that comprises the following steps:
a) Producing a mixture of a green coke, a base and a
hydrophilic polymer that is chemically inert with
respect to the base,
b) Compacting the mixture produced in step a) to form
a compacted pellet, and
c) Activating the compacted pellet produced in step
b).
With this process, it is possible to produce a
surprisingly activated porous carbon, in particularly
using green coke as well, that has exclusively or at
least almost exclusively micropores with the
characteristics profile described in the preceding. A
further advantage of this process consists in that the
formation and distribution of the reduction product of
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the base, such as vapour-phase potassium is effectively
avoided in the apparatus in which the activation is
carried out. This is firstly because a compacted
pellet, not a powder, is processed during and after the
activation, and the pellet has a low surface area per
weight compared with powder, with the result that no
potassium vapour escapes therefrom at the temperatures
that prevail during the activation. Secondly, the
addition of the hydrophilic polymer when the mixture is
being compacted results in the production of a dense
compacted pellet that remains dimensionally stable
particularly in the high temperatures that prevail
during the activation, because the polymer functions
surprisingly as a binding agent, that is to say it
binds the green coke particles and the base particles
together. Consequently, the compacted pellet is
reliably prevented from disintegrating even under the
high temperature conditions that are present during the
activation. The stability of the compacted pellets
enables the reagents to come into deep contact with
each other during the activation, which in turn assures
more intense reactivity and more of the base is used
during the activation, so that a comparatively small
quantity of the base needs to be used in this process.
Moreover, in this process the activation does not have
to be carried out in a gas stream such as a nitrogen
stream; instead, inertisation is assured automatically
during the activation by the gases from the pyrolysis
of the green coke and the hydrophilic polymer, so that
potassium vapour present in the apparatus cannot be
propagated in the apparatus. Consequently, it is
possible to avoid corrosion of the apparatus in which
the activation is carried out. A further advantage of
this process is the freely selectable size of the
compacted pellet, which lends the process a high degree
of flexibility. It is also possible in particular to
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produce very large panels by this process, which
enables the furnace chamber to be charged economically.
A further object of the present invention is an
activated porous carbon that is obtainable by the
process described in the preceding, that is to say an
activated porous carbon that is obtainable by a process
comprising the following steps:
a) Producing a mixture of a green coke, a base and a
hydrophilic polymer that is chemically inert with
respect to the base,
b) Compacting the mixture produced in step a) to form
a compacted pellet, and
c) Activating the compacted pellet produced in step
b).
As was explained in the preceding, a carbon that is
obtainable by this process has a specific BET surface
area between 1,400 and 1,900 m2/g, and contains
exclusively or at least practically exclusively
micropores with an average diameter between 0.3 and 0.9
nm, that is to say at least 80%, preferably at least
90%, more preferably at least 95%, especially
preferably at least 99%, and most preferably 100% of
all pores have an average diameter between 0.3 and 0.9
nm. Consequently, this activated carbon is
characterised by a high specific capacitance of between
130 and 150 F/g for example, and a high volumetric
capacitance of between 80 and 100 F/cm3 for example.
For the purposes of the present invention, the
hydrophilic polymer used in step a) of the process is
understood to be a polymer that is liquid at 23 C and
has rate of solubility in water at 23 C of 10 g/l, or
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a polymer that is solid at 23 C and has a contact
angle with respect to water of less than 900.
The term polymer for the purposes of the present
invention also includes oligomers as well as polymers
in the narrower sense.
For the purposes of the present invention, a polymer
that is chemically inert with regard to the base used
is understood to be a polymer that does not react with
the base, and in particular does not undergo
decomposition, particularly no chain shortening, if it
is in contact with the base for 24 hours at 200 C. The
chemically inert polymer also does not exhibit any loss
of binding properties if it is in contact with the base
for 24 hours at 200 C.
Process steps a), b) and c) are preferably carried out
immediately consecutively, that is to say with no other
intermediate steps therebetween, that is to say the
mixture produced in process step a) and also the
compacted pellet produced in step b) undergo process
steps b) and c) respectively without any intermediate
steps, particularly no dehydration and/or granulation
step. In this way, it is possible to produce activated
carbon having the previously described advantageous
properties simply, quickly and economically.
According to the invention, any hydrophilic oligomer or
polymer that is chemically inert with respect to the
base used may be used in process step a). Good results
are obtained for example if a polyether, or preferably
a polyether polyol is used as the hydrophilic polymer.
In a refinement of the inventive thought, it is
suggested to use a polyether polyol having the
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following general formula I as the hydrophilic polymer
in process step a):
H0(-R-0-)J1 (1),
wherein
n is a whole number between 2 and 100,000, preferably
between 2 and 1,000, and particularly preferably
between 100 and 600, and
R is a linear or branched-chain alkylene group,
substituted or not with one or more hydroxyl group(s),
preferably a alkylene group
preferably
substituted or not with one or more hydroxyl group(s),
and particularly preferably a Cl-C alkylene group
preferably substituted or not with one or more hydroxyl
group(s). All these polyether polyols are chemically
inert with respect to common bases and exhibit
sufficient hydrophilic properties for the purposes of
the process.
Particularly preferred polyether polyols according to
general formula I are those with a C1-C6 alkylene group,
substituted or not with one or more hydroxyl group(s),
the substances used as radical R are therefore selected
from the group including polymethylene glycol,
polyethylene glycol, polypropylene glycol, polybutylene
glycol, polypentylene glycol, polyhexylene glycol,
polyglycerins and any mixtures of two or more of the
cited compounds. Polyglycerins that are particularly
suitable for the purposes of the present invention are
such that have the following general formula II:
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OH
0
OH
OH
(II)
wherein
n is a whole number between 2 and 100,000, preferably
between 2 and 1,000, and particularly preferably
between 100 and 600.
According to a further particularly preferred
embodiment of the present invention, polypropylene
glycol and/or polyethylene glycol is used as the
hydrophilic polymer in process step a), wherein liquid
polypropylene glycol and/or polyethylene glycol and
= particularly polyethylene glycol with a weight-average
molecular weight (Mw) from 200 to 600 g/mol has proven
particularly suitable. If solid polypropylene glycol
and/or polyethylene glycol is used, it is preferably
used in the form of a fine powder having an average
particle diameter between 0.1 and 1,000 pm,
particularly preferably with an average particle
diameter between 0.5 and 50 pm, and especially
preferably with an average particle diameter between 1
and 10 pm, so that the solid polypropylene glycol
and/or polyethylene glycol may be mixed homogeneously
with the green coke. In keeping with the standard
definition of this parameter, the average particle
diameter is understood to be the d50 value, that is to
say the particle diameter value below which 50% of the
particles present fall, in other words, the particle
diameter of 50% of all the particles present is smaller
than the d50 value.
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Particularly in the case that a liquid hydrophilic
polymer is used in process step a), it is preferred to
mix the hydrophilic polymer with the green coke first,
before adding the base and mixing it with the mixture
produced in this way, in order to prevent the base from
dissolving in the polymer. An intensive mixer is
preferably used as the mixer for this purpose.
In general, all bases that lend themselves to an
oxidative chemical activation of carbon may be used in
process step a), wherein alkali metal hydroxides and
alkali metal carbonates are particularly suitable for
this purpose, such as preferably lithium hydroxide,
sodium hydroxide, sodium carbonate and potassium
carbonate, and most particularly potassium hydroxide.
If the base is solid at room temperature, which is
preferred, the base too is preferably added in the form
of a powder, wherein the average particle diameter of
the base is preferably between 0.1 and 1,000 pm, and
particularly preferably between 0.5 and 100 pm.
In principle, all types of green coke may be used in
process step a), that is to say all types of non-
calcined coke with 10 to 155'6 volatile fractions, such
as isotropic coke, electrode coke and needle coke,
powder-form green coke having an average particle
between 0.1 and 1,000 pm being particularly preferred.
The actually preferred particle diameter of the green
coke used in process step a) depends on the nature of
the subsequent application of the activated carbon. For
example, whereas average particle diameters of about
500 pm are preferred for its use as adsorption
material, if it is to be used as electrode material a
smaller particle diameter is preferred, particularly an
average particle diameter between 0.5 and 50 pm, and
particularly preferably an average particle diameter
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between 1 and 10 pm. If the activated carbon is to be
used in a double layer capacitor, the average particle
diameter of the green coke used in process step a)
should preferably not exceed 5 to 10 pm.
It has also proven advantageous for the purposes of the
present invention of the powder-form green coke used in
process step a) has no porosity, or only very low
porosity, less than 10 m2/g.
In general, the individual components may be used in
any ratio relative to each other in process step a),
although the degree of activation of the carbon is
adjusted via the base content, with the proviso that a
higher base content in the mixture produced in process
step a) results in the specific surface area of the
activated carbon being increased, whereas the
dimensional stability of the compacted pellet produced
. in process step b) is adjusted via the content of
hydrophilic polymer, with the proviso that a higher
polymer content results in greater dimensional
stability of the compacted pellet. For this reason, it
is preferred that the hydrophilic polymer constitute 3
to 10% by weight of the mixture, whereas the proportion
of green coke to base is preferably 1:1.5 to 1:2.
Taking these trends into account, in a refinement of
the inventive thought it is suggested to produce a
mixture in process step a) that contains 20 to 50% by
weight green coke, 1 to 15% by weight hydrophilic
polymer and 35 to 79% by weight base, preferably 25 to
40% by weight green coke, 2 to 10% by weight
hydrophilic polymer and 50 to 73% by weight base, and
particularly preferably 30 to 35% by weight green coke,
3 to 7% by weight hydrophilic polymer and 58 to 67% by
weight base.
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In a particularly preferred embodiment of the present
invention, the mixture produced in process step a)
contains 25 to 40% by weight green coke, 2 to 10% by
weight polyethylene glycol with a Mw from 200 to 600
g/mol, and 50 to 73% by weight potassium hydroxide, and
particularly preferably 30 to 35% by weight green coke,
3 to 7% by weight polyethylene glycol with a Mw from
200 to 600 g/mol, and 58 to 67% by weight potassium
hydroxide. Under these conditions, it is possible to
obtain activated carbon having a BET surface area
between 1,400 and 1,900 m2/g with the process.
In process step b) according to the invention, the
mixture produced in process step a) is compacted to
form a compacted pellet. For the purposes of the
present invention, a compacted pellet is understood to
a compacted body with a longest dimension, that is to
say in the case of an at least essentially spherical
compacted pellet the diameter, or in the case of a
polygon a length of at least 50 mm, preferably of at
least 100 mm, particularly preferably of at least 1 cm
and most particularly preferably of at least 10 cm. An
example of such is a cuboid compacted pellet having
both a length and a width of about 50 cm.
Generally, the compacting in process step b) may be
carried out using any suitable compacting pressure,
although it should be noted that as the pressure
increases so the density of the compacted pellet also
increases and the maximum furnace charge for activation
is thus increased. For this reason, the compacting in
process step b) is preferably carried out in such
manner that the mixture produced in process step a) is
compacted to yield a compacted pellet having a density
of at least 1 g/cm2, preferably a density of at least
1.25 g/cm2, particularly preferably a density of at
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least 1.5 g/cm3, and especially preferably a density of
at least 1.7 g/cm3.
For example, with a compacting pressure of 100 kg/cm2
it is possible to obtain a compacted pellet having a
density of about 1 g/cm3, whereas with a compacting
pressure of 5 tons/cm2 it is possible to produce
compacted pellets having a density of about 1.7 g/cm3.
For this reason, the compacting in process step b) is
preferably carried out in a die press with a pressure
of at least 100 kg/cm2.
The success of the heat treatment according to process
step c) depends primarily on the maximum temperature
reached during the heat treatment and the time for
which this maximum temperature is maintained. According
to the invention, the heat treatment of the compacted
pellet in process step c) is carried out at a maximum
temperature from 500 to 1,500 C, this being preferably
set to 700 to 1,000 C, particularly preferably 700 to
900 C, and especially preferably 850 to 900 C.
In this context, it is preferred that the maximum
temperature be maintained for at least 0.5 hour,
particularly preferably for at least 1 hour, especially
preferably for at least 2 hours, and most preferably
for at least 3 hours.
The preferred heating rate depends on the quantity of
material in the furnace, slower heating rates being
more appropriate for ensuring uniform heating of larger
material quantities than of smaller material
quantities. Depending on the quantity of material in
the furnace, generally good results are obtained if the
heating rate is 1 to 100 C/min, preferably 2 to 50
00/min, and particularly preferably 5 to 25 C/min.
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In a refinement of the inventive thought, it is
suggested to cool the compacted pellet to room
temperature quickly after maintaining the maximum
temperature in process step c), and this may be carried
out expediently by first cooling the compacted pellet
to about 150 C in the furnace before preferably
quenching it in water.
According to a further preferred embodiment of the
present invention the activated compacted pellet is
washed in a process step d) following the heat
treatment, in order to remove impurities from the
activated carbon. The washing operation preferably
includes at least one washing step with a mineral acid
such as hydrochloric acid or sulphuric acid, followed
by repeated washing cycles with distilled water until
neutrality is reached.
A further object of the present invention is the use of
the activated carbon described in the preceding as
adsorption material or an electrode, and preferably as
an electrode in a double layer capacitor.
